﻿242 SMITH. 



a high power the closely knit appearance of the mass almost leads to the 

 belief that this is a true igneous rock, but with lower power, when a 

 proper view of the whole can be obtained, its true texture is plain and it 

 is evident that the pseudo-igneous texture is due to the matrix of chlorite 

 and iron compounds which cement the larger grains. 



Remarks'. — I would call this rock a graywacke for the following reasons: 



1. Its composition. 



2. Its fabric, the rock consisting of an aggregate of more or less independent, 

 rounded and angular minerals, not rock fragments as in the true tuffs (see num- 

 bers 101 and 103). 



BBNGUET NO. 153. — QUARTZ DIORITB. 



Macroscopic. — This is a phanero-crystalline, medium-grained (with 

 finer grained inclusions) xenomorphic rock of a pepper and salt color. 

 The rock consists largely of more or less irregular and ill-defined feldspar 

 and ferromagnesian minerals, the darker minerals possessing more nearly 

 idiomorphism. In the white, dull feldspar patches there are minute 

 pinkish areas of orthoclase. The prcdominent feature about this rock 

 is the magmatic differentiation. The cryptocrystalline areas, usually 

 1 to 2 cubic centimeters across, are in every case darker than the 

 remainder of the rock and the border between the two areas is generally 

 sharp. 



Microscopic. — In the hand specimen this is not greatly unlike the 

 Kobe (Japan) granite. For the most part it is coarse grained and 

 possesses the typical granitic texture. In both rocks the characteristic 

 feature is the areas of darker, more fine-grained portions, which almost 

 have the appearance of foreign material caught up in the crystallizing 

 magma. This is the "Kugel Structur" of Eosenbusch and described by 

 him from the Corsican "Kugeldioriten von S. Lucia di Tallano bei 

 Sartene." (See p. 120, Eosenbusch, Massige Gesteine.) 



Two very distinct areas are seen under the microscope, which, however, 

 differ more in texture and abundance of certain minerals than in the 

 mineral composition itself. The same minerals appear in both areas, 

 but in different projtortions. The minerals found are : 



Primary. 



Plagioclase. 1 . , 



° /- Accessory : ophene. 



Hornblende. ) 



Magnetite. 1 



Quartz ] Secontlal T : Iron oxide. 



This makes the rock a quartz diorite. The quartz is not secondary, as 

 is shown by its relations to the other minerals, at least it is not the case 

 with all of it. 



Plagioclase. — This is the most abundant constituent of the slide, found 

 in large, idiomorphic sections, showing both albite and Carlsbad twin- 

 ning. Extinction angles on section showing albite twinning gave the 

 figures 20° to 22° -which would throw these plagioclases in with andesine. 



